The utilisation of industrial centrifuges depends to a large extent on the control equipment fitted to ensure that the degree of separation of the solids and liquid constituents of the feed slurry meets the process requirements in the minimum time and with the minimum use of resources (power, time, wash liquid, etc.). In addition the controls should provide data for centralised overall process optimisation. By ensuring that the centrifuge is fully loaded with feed slurry and then measuring accurately and continuously the volume of material in the rotating basket as the centrifuge cycle proceeds, adjustments to feed, wash, rpm, spin time, etc. may be made to optimise performance for that particular basket load under rotation—rather than rely upon preset mean values that remains unchanged for successive cycles. Where variations are inherent in the process (e.g. feed rate, solid/liquid ratio, solids wash, etc.) control adjustments are essential during each centrifuge cycle to achieve full process optimisation of each cycle independently.
FIG. 1 of the accompanying drawings shows a typical batch type centrifuge having a basket 1 supported on a drive shaft 2 and contained in a stationary outer casing 3. When the empty basket 1 is rotating, a feed valve 4 opens to allow slurry 5 to flow into the suspended rotary, perforated, open top basket and, under the centrifugal force of rotation, to form a cylindrical batch of slurry (i.e. the near cylindrical volume 6 on the inner basket wall) of radial depth (D). A perforated screen 7 covering the inner basket wall supports the solids but allows the liquid to flow to the outer casing 3 through the screen openings and perforations 8 in the basket wall, thus commencing the separation of the solids from the liquid. For illustration purposes, FIG. 1 shows a centrifuge with a suspended overdriven basket. The descriptions that follow apply equally well to under-driven, horizontal and inclined spindle centrifuges.
An existing method of closing the feed valve 4 by measuring the slurry depth (D) in the basket (and hence the slurry volume) uses a blade 9 mounted on a supporting arm 10 which in turn, is supported by and is free to rotate in an arc in a bearing 11 mounted on the outer casing top 12. When feeding slurry commences, the blade 9 is rotated to position (A) and, as the basket fills, rides on the surface of the slurry and is displaced to position (B) to operate a switch to close the feed valve. Position (B) is preset so that the inner surface of the slurry is approaching the basket lip 13 but set with sufficient margin (C) to avoid overflow of slurry over the basket lip.
During feeding, liquid flows through the screen 7 and perforations 8 as separation commences. After the movement of the blade 9 has been detected and the feed valve 4 closed, the liquid flow through the screen reduces the slurry volume and depth (D) in the basket to increase the dimension (C). It is advantageous on some processes to reopen the feed valve for a short preset time to add just sufficient extra slurry to compensate for the liquid separated so far—thus increasing the total amount of slurry processed. Again the short preset time is restricted by the limitations described in (c) below to avoid slurry overflow over the basket lip.
This existing method of feed control described above has operational limitations, including:                (a) To exert sufficient force to operate a switch (which in turn closes the feed valve 4), blade 9 is depressed below the surface of the slurry, introducing an error in depth measurement.        (b) This depression generates waves on the inner surface of the slurry which result in a measurement error and vibration and overflow unless allowance is made in setting position (B) to increase margin (C)—thus reducing the volume of slurry processed.        (c) For applications where the process parameters vary the rate at which the slurry flows to the basket, position (B) is set to avoid overflow in the “worst case” (i.e. highest slurry temperature, lowest viscosity, lowest solids content, etc.,). At these preset settings, the slurry fed to a centrifuge operating with parameters other than the “worst case” will be less than the optimum.        
An existing alternative method of closing feed valve 4 uses an ultrasonic retro-reflective system to measure the depth and volume of the rotating slurry cylinder 6. FIG. 2 of the accompanying drawings shows the part-section of a centrifuge basket, casing and casing top in which is mounted an ultrasonic unit 20 that extends into the basket interior. The ultrasonic unit comprises a sound generator 21, a sound receiver 22 and a sound reflector plate 23 mounted in a supporting tube 24 fixed to the casing top 12. The generator 21 produces a series of ultrasonic pulses directed along the tube 24 to reflect on plate 23 and the slurry surface (or basket inner surface) to return via plate 23 to the receiver 22 mounted close to, or concentric with the generator 21. The dotted line in FIG. 2 shows the path 25 taken by the sound pulses. By comparing the time taken for the sound pulses to travel over path 25 with and without slurry in the basket, the unit converts the time difference to a measure of the depth (D) of the slurry. As the depth (D) of the slurry fed increases and the margin (C) is approached the signal is used to close the feed valve.
This alternative method also has operational limitations, including:                (d) The velocity of sound in air varies with the air temperature, humidity and air movement, leading to an error en depth measurement with any change in these characteristics.        (e) Liquid droplets, vapours, steam and air movement in the basket all vary with the basket speed and diminish the strength of the sound pulses returned to the receiver 22. The disturbance and diminution increases sharply with basket speed, limiting measurements to low basket speeds.        (f) For applications where the process parameters vary, without the measure of the rate of flow of slurry being made, the margin (C) must be preset for the “worst case”—a limitation on process optimisation described in (c) above.        
A further existing method of closing the feed valve also uses an ultrasonic system, placing the sound generator 21 and sound receiver 22 inside the basket 1 in the position occupied by the reflector plate 23 which is not used. The ultrasonic pulses pass directly from the sound generator to the slurry surface and reflect back directly to the sound receiver. This method has the limitations given in (d), (e) and (f) above.
These prior art methods limit the slurry fed to the basket to less than the maximum by ensuring that the margin (C) is sufficient to avoid the overflow of slurry over the basket lip and to offset the limitations of the system. The penalty for an overflow is severe. Firstly the unseparated solids require reprocessing and may contaminate the separated liquid, and secondly the overflow causes basket unbalance, vibration and a centrifuge shutdown for the basket load to be rebalanced before the centrifuge cycle can proceed.
Furthermore, the methods described above control the closure of the feed valve 4 prior to acceleration and spinning for final separation and play no further part in the optimisation of the centrifuge cycle or the process after the feed valve closes.
It is an object of the present invention to provide a means for overcoming or at least mitigating at least some of the aforegoing shortcomings of the known systems.